The present invention relates to an aqueous autodepositable composition that is useful as a metal surface treatment.
It is well-known that metal surfaces are subject to corrosive and chemical degradation. This degradation has been combated by the application of various treatments to the metal surface. Conversion coating of the metal surface is one such treatment. Conversion coating generally involves treating the surface with chemicals that form a metal phosphate and/or metal oxide conversion coating on the metal surface. The conversion coating provides protection against corrosion and can enhance adhesion of any subsequent coatings. Phosphatizing is a well-established conversion process. However, phosphatizing suffers from several drawbacks. It is a complex multistep process that is capital intensive, requires close monitoring and can generate significant amounts of waste sludge. In addition, phosphatizing requires oxidative accelerators that promote corrosion and thus must be removed by multiple rinsing steps. Conventional inorganic phosphate conversion coatings are also very brittle and thus can fracture. A seal coat also is typically applied for good corrosion resistance that often includes hexavalent chrome which presents considerable environmental problems.
It is also generally known that the corrosion resistance of metal substrates can be improved by coating the substrate with an autodeposition composition that generally comprise an aqueous solution of an acid, an oxidizing agent and a dispersed resin. Immersion of a metallic surface in an autodeposition composition produces what is said to be a self-limiting protective coating on a metal substrate. The general principles and advantages of autodeposition are explained in a multitude of patents assigned to Parker Amchem and/or Henkel (see, for example, U.S. Pat. Nos. 4,414,350; 4,994,521; 5,427,863; 5,061,523 and 5,500,460).
U.S. Pat. No. 5,691,048 includes phosphoric acid in a list for possible acids in an autodepositing composition, but hydrofluoric acid is the preferred acid. This patent also lists hydrogen peroxide, chromic acid, potassium dichromate, nitric acid, sodium nitrate, sodium persulfate, ammonium persulfate, sodium perborate and ferric fluoride as possible oxidizing agents. Hydrogen peroxide and ferric fluoride are preferred.
Phosphatizing is also a well-known conversion treatment for providing corrosion resistance to metal surfaces. U.S. Pat. No. 5,011,551 relates to a metal conversion coating composition that includes an aliphatic alcohol, phosphoric acid, an alkali nitrate, tannic acid and zinc nitrate. U.S. Pat. No. 4,293,349 relates to a steel surface protective coating composition that includes pyrogallic acid glucoside, phosphoric acid, phosphates of bivalent transition metals such as Zn or Mn, Zn or Mn nitrate, and, optionally, formaldehyde.
An environmentally acceptable, user-friendly metal treatment with superior corrosion resistance and fracture toughness would be very desirable.